1. Field of the Invention
The invention relates to applying a heat curable liquid dielectric on a substrate to form a cured dielectric film, and more particularly to coating a heat curable liquid dielectric on a substrate and then curing the dielectric by increasing the temperature at most 15.degree. C. per minute until a predetermined cure temperature not exceeding 450.degree. C. is reached, thereby providing from the single coating a cured dielectric film at least 15 microns thick with a uniformity of less than 5 microns.
2. Description of Related Art
Integrated circuit chips can be mounted and interconnected on high density multichip modules or substrates, for instance by tape automated bonding. Once fabricated these substrates typically comprise a top surface with bonding pads or bumps, a dielectric layer with buried electrical conductor lines to interconnect the pads, and a base beneath the dielectric for physical support. As used herein, "substrate" refers generally to any material on which a dielectric layer can be coated.
There are several conventional techniques known for coating polyimide on a substrate including spin coating, meniscus coating, spraying and dipping. Spin coating is the most common technique for depositing thin polyimide films. It is well characterized and automated spin coating --equipment is widely available. For example, a pool of liquid polyimide can be deposited on a substrate and rotated at 2000 rpm. The polyimide is radially spun out and forms a substantially uniform layer above the substrate. There are, however, several drawbacks to spin coating. Spin coating normally tends to waste approximately 80-90% of the polyimide as it is slung off the edge of the substrate. The polyimide has a tendency to bead to a greater thickness at the edge of the substrate (edge bead). Nonuniformities arise across substrates. And difficulties arise with flexible or large area substrates. Furthermore, it may be desirable to have at least 20 microns thick polyimide to reduce the line impedance of thin conductive metal traces to about 50 ohms. Producing a cured polyimide layer 15-25 microns thick by spin coating a single uncured layer typically leaves 5-7 microns of uniformity. Where better uniformity is required, for instance 1-3 microns, it becomes necessary to perform 2 or 3 separate spin coatings. This reduces throughput and increases waste since a soft bake is normally required after each spin coating to increase the viscosity of the wet film before the next coat is spun on. For instance, "Pyralin (Trademark) LX----Polyimide Coating Processing Guidelines" by Du Pont, H-12294 (May 1989) describes a multicoat of Pyralin LX used to make one thick layer. After each coating the Pyralin LX is soft baked for 30 minutes at 100.degree. C. and then for 30 minutes at 175.degree. C. or less before the next coating is applied.
Curtain coating (which may encompass extrusion coating) appears to be a promising alternative to spin coating. Curtain coating can be performed by using a precision fluid dispense system to pump a curtain of liquid polyimide through a narrow precision linear orifice or slit preferably the same width as the width of the substrate. The substrate is passed under this "waterfall" of polyimide at a linear velocity. The linear velocity of the substrate may exceed that of extruded polyimide in order to stretch the polyimide film over the substrate. In either case the deposited layer of wet polyimide film typically is uniform and exhibits little or no edge bead. The polyimide wasted by curtain coating as compared to spin coating can be reduced drastically, e.g. by 50-90%. In addition, unlike spin coating, the entire top surface of the substrate need not be covered with dielectric. The resultant dielectric thickness depends on several parameters, such as the dispense rate, slit thickness, substrate velocity, polyimide viscosity, shrinkage factor of the polyimide during curing, and polyimide temperature.
Initial experiments performed by Applicant to deposit a single coating and apply a constant temperature cure to form a relatively thick (at least 15 microns) cured polyimide layer on porous substrates revealed several difficulties in the final cured polyimide. Runs, streaks, crater-like texture, and separation from the substrate were observed.
Similar problems have been noted in the related art. For instance, "Polymers For Electronic Applications," edited by J. H. Lai, CRC Press, p. 40 (1989) points out that the largest practical film thickness achievable by spin coating is on the order of 15 microns; thicker films, such as required for interlayer dielectrics in integrated circuit packaging applications, are deposited with multiple coatings with a cure temperature of at least 150.degree. C. between coats. Likewise, U.S. Pat. No. 4,395,527 by Berger entitled "Siloxane-Containing Polymers" indicates that a solution of a half-amide solvent can be applied to coat a substrate and then heated to evaporate the solvent and convert the half-amide into the corresponding polyimide. However, the coating should be built up gradually from successive layers of very thin coatings. If a thick coating is applied and heated to effect conversion of the half-amide to the imide, the water formed during the reaction can be converted to steam which tends to produce voids (broken continuity) in the coating. Further, as the imidization proceeds from the surface downward, the bottom of the coating is insulated from the heat and this makes completion of the imidization reaction difficult to achieve, or even to identify. The '527 patent then concludes that it is necessary for the user to engage in a series of coating, heating, and cooling cycles in order to develop an acceptable coating.
In addition, the related art typically describes curing the polyimide at a constant temperature. For example, in "Pyralin (Trademark) LX Polyimide Coatings----Technical Data Sheet For PI-2610D and PI-2611D" by Du Pont, H-12295 (April, 1989) the polyimide is made solid by first applying a soft bake (prebake) for 3 minutes at 140.degree. C., and then a cure for 30 minutes at 350.degree. C.
Therefore the related art does not teach how to fabricate from a single coating cured dielectric film at least 15 microns in thickness with a uniformity of less than 5 microns that adheres well to the substrate.